Ironically, modern technology had much to do with the creation of indoor air quality concerns. Outdoor air infiltration and high rates of outside make-up air were acceptable when energy was relatively cheap. Energy became more expensive in the early 1970's and architects and engineers responded with designs for tightly sealed, energy-efficient buildings. Indoor air is now recirculated and kept indoors much longer than before. Furthermore, the average American is now spending greater than 90 percent of their time in indoor environments.
There are indoor environments that appear to be causing health and comfort related problems to building occupants and the causes are not identifiable. Pollutant compounds that may be responsible are either not being identified or are not being considered. Common symptoms associated with Sick Building Syndrome (SBS) can include stinging, itchy eyes; throat and nasal membrane irritation; and the perception of odors. Such sensory irritations may lead to headaches, fatigue, general lethargy and worse.
Decades-long research on the adverse effects of outdoor air pollutants has demonstrated that increased concentrations of small particles are directly related to increased mortality. Furthermore, recent studies by researchers at the Harvard School of Public Health suggest that indoor particles may be more bioactive (i.e., potentially unhealthy) than outdoor particles. Therefore, there is no question as to the necessity of employing a good particulate filter in the effort to improve indoor air quality.
It must be understood that the indoor environment is very different than the outdoor environment when it comes to air pollution. Because of the enclosed environment and recirculated conditions, pollutants are trapped together for an extended period of time and chemical reactions can take place between pollutants that are normally rapidly dispersed in outdoor environments. This phenomenon can work to intensify the concentration of irritable and sometimes toxic pollutants. This is of great concern with the use of some types of air freshening and “purifying” devices that can actually aid in chemical transformations, such as ionic devices and devices that promote rapid oxidation of chemicals.
New studies undertaken in an attempt to identify pollutants that pose health risks have lead to the conclusion that there is no simple answer. Many of the various pollutant compounds are found in very low concentrations that are difficult to analyze with today's analytical and instrumentation technologies. Furthermore, the adverse health effects from exposure to these compounds, especially multiple compounds at the same time, are difficult to explain. What this means is that finding and eliminating the source of an irritant or toxic compound is not always feasible.
One of the best monitors available for identifying and gauging the reactivity of indoor air pollutants is the human nose. We perceive the world by way of five inputs: seeing, hearing, touching, tasting and smelling. Smell is unique in that it is the sense that is always open to stimulation. Wherever we are we may close our eyes, plug our ears, touch and taste nothing, but we must breathe, and in drawing in air, we stimulate the base of our brains.
Smelling is a complex operation. New research has discovered nearly 1,000 human genes that are dedicated to the sense of smell. Throughout the history of humans, staying alive often meant being able to interpret smells correctly, whether it was identifying rotten food, or being attracted to a sexually alluring musk. Our noses and brains are programmed to analyze and detect the natural environment. The variety of “olfactants”, the molecules we can smell, is near 10,000, some at surprisingly low concentrations, as shown in Table 1 below.
TABLE 1OlfactantSensitivity ThresholdConcentrationmusk0.000,000,000,004 4 parts per trillionrancid fat0.000,000,000,060 60 parts per trillionvanilla0.000,000,000,080 80 parts per trillionrotten eggs0.000,000,180180 parts per billionformaldehyde0.000,000,100100 parts per billionchlorine0.000,004 4 parts per million
In evaluation of current monitoring technologies and studies performed in an attempt to identify problem compounds that contribute to SBS, it is apparent that it may not be possible to come to any specific and useful conclusions. Furthermore, because of the nature of many of the pollutant compound sources, such as building materials, furnishings, cleaning equipment, electrical equipment, building occupants, infiltration of outside air, etc., it may not be feasible to easily eliminate the source of the problem.
As described in many U.S. Environmental Protection Agency reports, indoor air pollution, both in commercial and residential environments are of increasing concern. Of primary concern are very fine particulate matter less than or equal to 2.5 microns in diameter (PM2.5) and volatile organic compounds (VOCs). Such pollutants contribute to indoor air quality concerns that affect building inhabitants in the forms of sick building syndrome, asthma, allergies, malodorous compounds, and general discomfort.
Sources of indoor air contaminates include, but are not limited to, entrainment of ambient outdoor pollutants through circulation of make-up air, poor HVAC design leading to unbalanced conditions that promote growth of mold and bacteria, off-gassing of natural and synthetic building materials, and occupant activities and emanations.
A wide variety of filtration media currently exists and is available for commercial and private use. The variety of existing filtration media includes various adsorbent media. However, the existing manufacturing methods used to produce adsorbent filtration media incorporate some form of binding or adhesive composition to affix various adsorbent compositions to said media. The use of binding or adhesive compositions lowers the efficiency of applied adsorbent compositions and furthermore, the degradation of binding and adhesive compositions through the normal life of the filtration media results in off-gassing and oxidation of some binding and adhesive compositions which can contribute to air borne pollution.
Additionally, through design, currently available adsorbent filtration media is generally targeted at only removing gaseous chemical compounds. For removal of particulate matter, a second filtration media specifically designed for particulate capture is needed.
It is also becoming apparent that there is a need to reduce concentrations of a wide variety of gaseous chemical compounds in the indoor air. It is not practical or possible to remove all sources generating gaseous chemical compound emissions, and even if one could, one would not know which compounds, or group of compounds, may be the source of the problem.
Gaseous chemical compounds, not unlike particulate matter, do have a size. However, that size is generally measured in units of angstroms ( 1/10,000,000,000 of a meter). In general they are 1,000 to 10,000 times smaller than a fine dust particle and can easily pass through the finest particulate filter. Therefore, there is a need for gas-phase filtration.
Physical adsorption, also referred to as van der Waals adsorption and adsorption condensation, has been widely accepted and proven as a technology for use in the removal of gases and vapors from contaminated air. It is well known that this process works by the weak bonding of gas molecules to a solid adsorbent. The bond energy is similar to the attraction forces between molecules in a liquid.
Some adsorbents used for air pollution control include activated carbon, alumina, bauxite and silica gel. Activated carbon is by far the most frequently used adsorbent, and has virtually displaced all other materials in solvent recovery and purification systems.
Standard activated carbon adsorption systems were initially developed for control volatile organic compounds (VOCs) from industrial process equipment. However, to work effectively, these systems generally contain very large quantities of activated carbon in a thick “carbon bed” system or a “packed tower”. The reason for the large container or thick carbon bed is to provide a long enough residence time for contaminated air flowing through the system to allow the VOCs to be captured and to provide enough adsorbent surface area.
When using standard activated carbon, which is generally in a granular form that is greater than 50 microns in diameter, it is difficult to adapt to a system for the light commercial and home environment. Most commercial and home environments do not have the room or cannot support the cost to operate an extensive industrial adsorption system.
Attempts have been made to produce a filtration media that consists of a thick layer of activated carbon that is glued onto, or sandwiched in between various types of media or substrate. The problem with these systems is that due to the large size of granular carbon particles, the thin layer of carbon has many holes that allow a contaminated air stream to pass through without coming into contact with the carbon, especially with low concentration air steams. Use of a thicker layer of carbon becomes prohibitive due to the inability to efficiently hold the carbon to the substrate. Furthermore, when adhesives are used to affix the carbon to a substrate, much of the effective adsorbent surface area of the carbon is reduced due to blinding by the adhesive. Adhesives also result in off-gassing of chemical compounds which can contaminate and shorten the life of the carbon.
By far, the major drawback to the use of larger granular carbon is the very low concentration of target pollutants in the indoor environment. When adsorbing a higher concentration gas with typical granular activated carbon, a gas will first diffuse onto the surface of the carbon and thus be trapped. However, there is a secondary diffusion phenomenon wherein the gas moves further into the adsorbent particle center, or void spaces, in essence, pushed into the void spaces by more gases waiting to be adsorbed to the surface. However, with low concentration gases, this secondary diffusion phenomenon does not take place because there are no high concentration gases there to push the first gases deeper into the adsorbent.
Because of this process, a very large surface area is needed for low concentration gases. Use of granular adsorbents does not provide enough surface area to maintain effective and efficient control of low concentration gases.
As such, what is needed is an improved filtration media, through the application of adsorbent compositions, without the use of binding or adhesive compositions.